1. Field of Invention
This invention relates to the separation of waste ingredients from a mixture of materials, including, but not limited to, mixtures of waste materials, specifically by aerating the mixture in a water-filled floatation container.
2. Description of Prior Art
Prior to about 1960, the disposal of waste material was of little concern to the public at large. Household garbage was dumped at any out-of-the-way spot on land that was not needed for some other purpose. Leaves and other yard wastes were simply burned at the generation site or in the street, instead of being composted. Apartment building operators in large cities simply burned their garbage in an incinerator located in the building. Both the soot and the odors were very noticeable. However, over the past thirty-five years, the generation of waste materials has been viewed as both a costly disposal nuisance and as an opportunity to recover some value from reuse of the waste materials. The incentive to find uses for the components of waste has been further enhanced by recognition that when all or a portion of a waste material can be used again, added value is also gained by avoiding the alternative cost of disposal. The most effective uses of the waste ingredients contained in mixed waste materials, such as Municipal Solid Waste (MSW), require that at least a partial separation of these waste materials be effected. For example, even if the primary reuse of the ingredients is as fuel for the purpose of generating energy, a better result is achieved if a large portion of the non-combustables and water are removed prior to the remainder being used as fuel. Where the ingredients of a waste mixture are to be reused for the same or another similar beneficial use, the value of the separated components are increased in accordance with the completeness of the separation. For example, metals designated for recycling should be free from other contaminants and, likewise, fibers which are to be reconstituted to new higher-value paper-like products need to be cleaner than when the same fibrous materials are used as raw fuel products. Also, the more completely that individual ingredients can be separated from mixed waste materials, the lower the disposal cost of residuals. For example, even though non-metallic inorganic waste material, such as sand and grit, may have a low intrinsic value, if that material can be made free of prutricables and organic material in general, it can be used for low-cost construction fill, rather than having to incur the alternative higher cost of sanitary landfill disposal. It is understood that landfills are not permanent disposal options. Many old landfills are leaking toxic materials to the groundwater, which has required that they be excavated and processed a second time for disposal. Thus it can be seen that there is an economic incentive to develop and employ improved methods of separating waste materials. This incentive has been increasing at a rapid rate in recent years, because disposal costs to sanitary landfills and mass-burn incinerators have increased disproportionately to the general economy, due primarily to more stringent environmental regulations.
A comprehensive review of past art, from publications and from patents that have been granted, shows that a wide range of methods have been employed for the separation and removal of waste materials from mixed wastes, including but not limited to Municipal Solid Waste (MSW). These methods can be categorized as follows: 1) manual separation, 2) screening, either before or after comminution, 3) contact with an upwardly moving air stream, 4) electromagnetic and optical, and 5) floatation in a liquid medium, including water. These methods have been employed both singularly and as systems which combine two or more methods.
In addition to the manual separations performed by MSW generators (i.e. the general population) through the mandatory curbside recycling programs, many commercial systems also rely on manual separation as the primary method of MSW processing for the recovery of specific ingredients. For example, U.S. Pat. No. 4,859,211 to Moore discloses a process in which MSW is manually sorted to separate materials of different value and degrees of contamination, including a portion which is processed to produce a fuel pellet. Likewise, U.S. Pat. No. 5,250,100 to Armbristor discloses a manual sorting technique wherein a large portion of the separated waste is directed to a composting operation. Manual sorting can provide very clean separations if performed at the source by the individual generators; however, this method causes high collection costs and places unnecessary burdens on the population. Manual separations of wastes after they have been co-rmingled (allowed to mix) are very labor intensive, generally expensive, viewed by some as a demeaning activity, and are not very effective in dealing with the removal of cross contamination (i.e. providing clean products).
The application of physical screening to effect MSW ingredient separation is based on experience that the non-combustable ingredients in MSW tend to be of smaller physical size than combustables. For example a piece of glass or dirt is usually smaller than a sheet of newspaper or cardboard. Therefore, if after the application of some mechanical action to expose the components of bagged material, the entire MSW is screened, the fraction which is composed of larger sized pieces will have a lower non-combustible content than the other fraction. The fraction containing the larger components can then be further processed to produce a refuse derived fuel (RDF) or some other beneficial use. For example, U.S. Pat. No. 5,101,977 to Roman discloses a multiple screening approach to MSW separation. Likewise, U.S. Pat. No. 4,479,581 to Kelyman, U.S. Pat. No. 4,561,860 to Gulley et. al., and U.S. Pat. No. 4,778,116 to Mayberry all disclose mechanical screening as a key component of their MSW processing systems. While it is true that selective screening will generally result in two fractions with different ratios of combustible to non-combustible fractions than the starting composition, unless further processing is employed, both portions separated by the screening method are generally only useful for low value applications, such as low-quality (dirty) fuel and compost.
Many of the largest commercial MSW separation operations have utilized some sort of air floatation apparatus for the separation of useable ingredients. For example U.S. Pat. No. 3,524,594 to Anderson et al, U.S. Pat. No. 3,738,483 to MacKenzie, U.S. Pat. No. 3,836,085 to Brown, U.S. Pat. No. 3,925,198 to Eckoffet al, U.S. Pat. No. 4,070,203 to Nollet, U.S. Pat. No. 4,210,527 to Patterson et al, and U.S. Pat. No. 4,264,352 to Houser all disclose apparatuses that use an upward flow of air to separate the ingredients of mixed waste into two or more portions. This method is very effective for some separations, such as separating a chunk of glass or a heavy metal object from lighter materials, such as paper and plastic items. The two principal disadvantages of air-float separation (where the objective is specifically to separate organic from inorganic materials) are that this method 1) does not effectively prevent buoyant inorganics, such as aluminum foil, and imbedded dirt from being carried along with the desired organic-rich portion, and 2) the heavier particle inorganic residue portion has a high organic content which makes it unsuitable for such applications as construction fill.
Electro-magnetic devices are commonly used to separate metals from mixed waste. Iron and most grades of steel can be extracted from a mixture of materials with a conventional magnet. Following the removal of magnetic materials, other electrically conductive material, such as aluminum, copper and non-magnetic stainless steel, can be removed using an eddy-current separator. These devices are very effective in removing metal from mixtures of other materials; however, they have no effect on the separation of non-conductive materials, such as the removal of sand and glass from paper, plastic and other organic wastes. These electromagnetic devices are usually used in combination with other separation methods. For example, U.S. Pat. No. 5,341,935 to Djerf et al and U.S. Pat. No. 5,387,267 to Warf disclose magnet separation followed by eddy current separation to remove metals from mixtures of waste materials. Optical scanners have been used, with apparent limited success, to separate glass from other mineral materials.
A summary of the commercial application of the above referenced approaches to waste separation methods has been provided by the U.S. Department of Energy in a Argonne National Laboratory publication, Practical Applications of Sizing, Separation, Homogenization and Densification Equipment in the Refuse-Derived Fuel (RDF) Industry, Oscar O. Ohlsson, et. al., presented at the AIChE 1992 Summer National Meeting, August, 1992.
Floatation in liquids, including water, is one of the oldest recorded methods of separating materials having different specific gravities. For example, U.S. Pat. No. 1,298,577 to Sawyer et al in 1919 discloses the separation of gold from sand using a device that allows heavier gold to settle out of a moving water stream, in which the sand remains suspended. Likewise, U.S. Pat. No. 1,548,971 to Ziska in 1925 discloses the separation of rocks from wood chips by using an apparatus which subjects the mixture of wood chips and foreign particles, such as rocks and nails, to an upward flow of water. The inventor, Ziska, specifically notes in his patent that an upward flow of water is necessary to prevent water-soaked wood chips from sinking along with the foreign particles that are being removed from the bottom of the floatation apparatus. Since the granting of U.S. Pat. No. 3,159,353 to Atwater in 1964, there have been numerous patents issued which describe modified approaches to the separation of mixed waste materials using liquid floatation methods. Examples of U.S. Patents, which employ water floatation to separate the ingredients in Municipal Solid Waste, include U.S. Pat. No. 3,568,839 to Dunlea, U.S. Pat. No. 3,597,308 to Brooks, U.S. Pat. No. 3,897,215 to Davidson et al, U.S. Pat. No. 4,250,023 to Samis et al, and U.S. Pat. No. 5,387,267 to Warf. In each of these cases the stated objective is to obtain a complete separation of organic material, including paper, textiles, wood, plastic and food waste from inorganic materials, such as metals, glass, sand and dirt in order to produce a low ash content fuel or compost material. However, unlike Ziska in his method for cleaning wood chips, none of these disclosures, with the exception of Brooks, recognize that most fibrous organic materials will also sink in water (when they become water-soaked) along with many types of plastic, unless some method of increased buoyancy is provided. Brooks recognizes three specific gravity categories associated with float separation in water; however, he uses a screw device rather than an upward flow of water to increase the buoyancy of organic materials having a higher specific gravity than that of water. Warf, on the other hand, asserts that all organic materials in mixed waste float while the inorganic materials sink, which is not usually the case. The incomplete separation, which results from traditional water-floatation methods, will both reduce the total recovery of useful organic materials and cause the sunken inorganic residue to be contaminated by organic material, such that it may require expensive landfill disposal rather than beneficial construction fill use.
Another approach to the use of water to separate fibrous organic materials, such as paper, from undesirable inorganic material is to first comminute or disintegrate the fibrous material by the high-speed wet shearing action of a hydro-pulper. The disintegrated material, along with only small particles of grit, are extracted in slurry form through the small holes of a perforated plate in the bottom of the hydro-pulper. Most of the remaining grit in the paper slurry can then be removed by conventional liquid cyclones. These liquid cyclones separate high specific gravity particles from the remaining mixture of water and lower specific gravity materials. If the hydro-pulper feed contains materials that do not disintegrate by the wet shearing action, such as the metals, plastic and glass (such as found in MSW), these materials will not pass through the perforated plate of the hydro-pulper and are periodically removed from the main chamber of the hydro-pulper. This method of separating waste materials was first disclosed in U.S. Pat. No. 3,549,092 by Baxter and assigned to the Black Clawson Company. Subsequently several other patents disclosing modifications and improvements of this method have been granted and assigned to the Black Clawson Company. These include: U.S. Pat. Nos. 3,595,488 and RE28,677 to Blakey et al. and 3,945,575, RE29,156, and 4,049,391 to Marsh. U.S. Pat. No. 4,026,678 to Livingston also includes hydro-pulping as a part of a process to produce fuel pellets. Commercial operation of this method by the Black Clawson Company is described in a 1974 EPA publication Recovering Resources from Solid Waste Using Wet Processing, EPA 530/SW-47d and on pages 225, 226 and 305 of the book Solid Waste: Engineering Principles and Management Issues, authored by George Tchobanoglous, et al, and published by McGraw Hill in 1977. The use of a hydro-pulper as a first step in a waste separation process is also described in a magazine article, "The Next Generation of Waste-to-Energy", Solid Waste Technologies, pg. 40, September/October 1997. It can be noted from these literature references that in the commercial application of this wet comminution method, no separation of waste materials was employed prior to the waste material being fed to the hydro-pulper. The fiber separated from Municipal Solid Waste (MSW) by this process in the Black Clawson commercial demonstration operation was used primarily as the paper mat foundation for asphalt roofing shingles. By using multiple liquid cyclones after the hydro-pulping, a very clean, low-ash fiber was produced. However, because there was no separation of abrasive materials, such as glass and metals, prior to the hydro-pulping operation, reduced capacity and high maintenance costs of the hydro-pulper were incurred. A hydro-pulper operates like a giant under-the-sink garbage disposal, which can be progressively damaged by including hard materials, such as glass and metal, with the food waste.